The invention relates to a method for operating a power steering system, to such a power steering system, to a control unit, and to a computer program and a computer program product for carrying out the method.
Power steering systems, which are also referred to as power-assisted steering systems, are used to achieve a reduction in the force that the driver must apply for steering. When using electric power steering (EPS) as the power-assisted steering system, a motor torque is computed depending on various input variables, the motor torque being superimposed, as added torque, on the manual torque that is applied by the driver.
So as to limit potential impact from faults in EPS systems, additional measures are known, which hereafter are referred to as safety measures. To this end, monitoring of the combination of manual and torque motor torque can be used to prevent safety-relevant operating situations by providing for a limitation to allowed combinations in the function path.
The published prior art DE 10 2009 000 165 A1 describes a method for operating a power steering system, in which a motor torque is computed by an electronic processor and established by suitable motor actuation. A plausibility check of the computed motor torque is carried out within the framework of the safety design, which is represented by a three-level design, for example. Contributions above the motor torque limit curve are integrated, and an integrator is decremented by contributions below the motor torque limit curve. The target motor torque is limited in level 1 to a first integration threshold. The limitation is monitored in level 2. Additionally, an intrinsically safe motor torque limit curve is used, as a function of an actual torque.
Over the past few years, special functions have been further developed, which utilize the EPS system by way of torque superposition, so as to achieve added value in terms of driver assistance. Examples include a parking steering assist function or a lane-tracking assist function (lane departure warning and lane maintenance support). As the degree of assistance rises, increasingly greater superimposed steering torques are required. These functions alter the manual torque depending on particular situations. Consequently, the combination of manual torque and motor torque is also altered, which has not been taken into account in the existing design.